1. Field of the Invention
The present invention relates to an ischemia detector and to an implantable heart stimulator having such an ischemia detector.
2. Description of the Prior Art
The blood flow and penetration in the circulatory system of a living subject are dependent on the arterial muscular tension, the so called tonus. Thus the blood flow is controlled by the tonus, the driving force of the flow is the blood-pressure in the elastic aorta and the pressure in the aorta is maintained by the pumping action of the heart. For this pumping action the heart needs energy in the form of oxygen and glucose. About 60% of the oxygen in the heart interstitial fluid is consumed within one heart beat. If the energy supplied to the heart is disturbed the heart contractibility and the pumping action of the heart are severely deteriorated and an oxygen shortage or ischemic situation will rapidly develop. Ischemia results from insufficient blood flow through the heart muscle. Due to blocking or passage congestion of coronary blood vessels of the heart. I ischemia is experienced by the patient as a severe chest pain and is one of the most stressing factors known to the organism. Several techniques for detecting ischemia are known. In U.S. Pat. No. 5,156,148 a system for treating a malfunctioning heart, e.g. ischemia, is known using the variation of selected physiologic parameters. In U.S. Pat. No. 4,821,735 a method and an apparatus for detecting myocardial ischemia are described, wherein the systemic vascular resistance (SVR) in a subject is monitored and the presence of myocardial ischemia is detected when the SVR increases by at least 60% over a base line value.
In U.S. Pat. No. 5,497,780 an apparatus is described for determining an ischemia by measurements of electric potentials between at least three implanted measuring electrodes, two of these electrodes being implanted with their poles in the heart and the third electrode being implanted with its pole lying outside the heart.
In U.S. Pat. No. 5,199,428 a technique is described for detecting ischemia and effecting stimulation of nerves regulating blood pressure and heart rate to reduce the heart's oxygen requirements while providing pacing therapies to maintain the patient's heart rate within acceptable limits to avoid bradyarrhythmias and/or unphysiological AV delays induced by the nerve stimulation. The ischemia detection is based on the occurrence of changes in the ST-segment variation different from pre-determined or programmed threshold levels, or on changes in the pH and/or in the dissolved blood oxygen in venous return blood in the coronary sinus region of the patient's heart.
An ischemic state can also be detected by analysis of recorded IECG's or surface ECG's to determine the heart rate variability. Ischemia can be detected by a lead bend sensor located at the distal end portion of an implanted heart stimulator lead. As the heart wall is thickening and stiffening as the result of an ischemic state, the accompanying change in the moving pattern of the heart wall can be detected in this way. Also sound absorption is affected by changes in the stiffness of the heart tissue and by measuring the absorption of sound waves, generated e.g. at the heart valve closure, on their way from the upper portion of the ventricle to the apex region, an ischemic situation can be detected. Ischemia deteriorates the efficiency of the heart's pumping and an ischemic situation can therefore be detected by studying blood pressures and cardiac outputs, too. Thus, by measuring the difference between the systolic and diastolic pressures and comparing this difference obtained from one heartbeat to the difference obtained from the next heartbeat an ischemic can be detected. An ischemic state can be detected using a flow sensor for measuring cardiac output, as well.
As mentioned above an ischemic state is normally associated with severe pain forcing the patient to sit down or lie down with a reduced heart rate as a consequence. At the same time the patient feels a need for forced breathing, so called hyperventilation.